This invention relates to an electric discharge system and, more particularly, it relates to an electric discharge device wherein an electrode and a workpiece are arranged to face each other through an insulating working liquid and an electric discharge is generated in an interpole gap to process the workpiece.
FIG. 1 is a schematic structural view of a conventional electric discharge device. In FIG. 1, an electrode 10 is positioned across a workpiece 14 placed in a processing tank 12 having an insulating working liquid 16. The output of a power supply 18 is connected across the electrode 10 and the workpiece 14. The power supply 18 comprises a DC supply 18a, a switching element 18b to establish an intermittent working current flow, a current-limiting resistor 18c, and an oscillator 18d for controlling the operation of the switching element 18b to supply current intermittently to the interpole gap 20 between the electrode 10 and the workpiece 14. The current I in the conventional electric discharge device is given by the equation I=(E-V.sub.g)/R (where E is the voltage of the DC supply; R is the resistance of the current-limiting resistor 18c; and V.sub.g is the interpole voltage). The interpole voltage reaches 20-30 V during discharge, O V when there is a short, E V without discharge, and O V when the switching element 18b is OFF. If the interpole voltage V.sub.g is detected and approximated by a smoothing circuit 22, the magnitude of the interpole gap may be controlled using the voltage. In other words, discharge does not generally occur when the interpole gap 20 is wide and thus the detected voltage V.sub.s is high. When the interpole gap 20 is narrow, the detected voltage V.sub.s is decreased, because short-circuiting or electric discharge readily occurs. Accordingly, if the difference between the detected voltage V.sub.s and the reference voltage V.sub.r obtained by comparison is amplified by an amplifier 24 and inputted to an oil servo coil 26, the electrode 10 may be controlled. The interpole gap 20 can be maintained roughly constant by means of an oil hydraulic servomechanism comprising an oil hydraulic pump 28 and an oil hydraulic cylinder 30.
In order to examine the quality of the processing condition in the conventional electric discharge machine, detected voltage V.sub.s obtained from the above interpole voltage V.sub.g is normally observed. When the detected voltage V.sub.s is low, the interpole impedance is also low and resulting in continuous arc discharge as well as short-circuiting. Consequently, dust incidental to processing and sludge are expected to accumulate in the interpole gap 20. In electric spark processing, the most dangerous irregular arc discharge is an electric discharge across carbon build up and the workpiece, the carbon is produced by the thermal decomposition of the working liquid. This condition establishes a state similar to what is created by a high interpole impedance. For this reason, the disadvantage is that it is impossible to detect, through the observation of the detected voltage V.sub.s, a deteriiorated state of interpole gap caused by the irregular arc discharge.